During the preseason:

1 Daric Barton, 1B, Grade A-
2 Kevin Melillo, 2B, Grade B+
3 Javier Herrera, OF, Grade B
4 Cliff Pennington, SS, B
5 Travis Buck, OF, B
6 Danny Putnam, OF, B
7 Santiago Casilla (Jairo Garcia), RHP, Grade B (changed from B+ book grade due to Age-Gate)
8 Craig Italiano, RHP, B
9 Shane Komine, RHP, B-
10 Jimmy Shull, RHP, B-
11 Jared Lansford, RHP, B-
12 Kurt Suzuki, C, C+
13 Vincent Mazzaro, RHP, C+
14 Richie Robnett, OF, C+
15 Dan Meyer, LHP, C+  
16 Jared Burton, RHP, C+
17 Jason Ray, RHP, C+
18 Ramon Alvarado, OF, C+
19 John Rheinecker, LHP, C
20 Justin Sellers, SS, C

I think that your grades are definitely fair at  this point.  The A's are in a world of long term hurt if they don't beef up the minor league system.  Injuries have killed the the seasons of the A's top two prospects.  Only Buck and Windsor have made a strong move forward.  Pennington looks like a complete bust(it is early but he was a college guy) and Melillo is looking more like Keith Ginter than the next Marcus Giles.  

John is very level headed, he obviously knows that 2 months does not make a season. So people such as Danny Putnam, i don't see their grade falling so much. Pennington might be a victim of that.

Here's what i think the grades are now (not in order though):

Daric Barton, 1B, Grade A-
Kevin Melillo, 2B, Grade B-
Javier Herrera, OF, Grade C+
Cliff Pennington, SS, C
Travis Buck, OF, B+
Danny Putnam, OF, C+
Santiago Casilla (Jairo Garcia), RHP, Grade B (changed from B+ book grade due to Age-Gate)
Craig Italiano, RHP, B
Shane Komine, RHP, C+
Jimmy Shull, RHP, C+
Jared Lansford, RHP, B
Kurt Suzuki, C, B-
Vincent Mazzaro, RHP, C+
Richie Robnett, OF, C+
Dan Meyer, LHP, C
Jared Burton, RHP, C+
Jason Ray, RHP, B
Jeremy Brown, C, C+ (replaces Rheinecker)
Landon Powell, C, C+ (replaces Alvarado-traded)
Justin Sellers, SS, C
Jason Windsor-B

More to add:

Matt Allegra-C+
Ben Fritz, C
Vasili Spanos, C+

There are more borderline C+ prospects, but i'm too lazy to add them right now